The present invention relates to harmonic signal reduction, and more particularly to harmonic signal reduction in the output fundamentals of a multi-bridge three-phase power supply.
Present day three-phase converter systems provide for two way conversion of electrical energy, selectably either from DC to AC or from AC back to DC, i.e. each converter functions alternately as an inverter (DC to AC) or a rectifier (AC to DC). As known, three-phase converters providing output power in excess of a few hundred kilowatts require the use of multiple converter bridges connected in parallel between the DC or AC sources and the load due to the limited power handling capability of individual bridges. When used as an inverter, each bridge provides at each of three-phase outputs, or poles, a square wave fundamental signal for each of the three output phases of the converter. The square wave fundamental signals from each bridge have a harmonic content which causes the power level of the bridge fundamental signals to change each time a pole switches. These harmonics may be determined from the Fourier series expansion for a square wave fundamental signal of frequency wt. as: ##EQU1## where V is the signal voltage magnitude and 0 the phase angle. Only odd harmonics are present in the expansion, and for a square wave fundamental signal measured between the output poles of the three-phase system there is inherent cancellation of the third harmonics and multiples thereof; i.e. triplens, such that the signal harmonics of the fundamental signal between pole outputs are odd harmonics less triplens, i.e. the 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37. . . etc.
As known to those skilled in the art, if two signals (current or voltage) are added to each other at an angle 360.degree./2N=180.degree./N, the Nth harmonics of each signal are displaced from each other by (180.degree./N) N=180.degree. and will cancel if the amplitudes of each are equal. If pairs of signals are added from two sets of P phase signals this angle becomes 360.degree./2PN=180.degree./PN, or 60.degree./N for a three-phase system. This relationship is used in providing harmonic cancellation in a two-bridge three-phase inverter as disclosed by Udo H. Meier in U.S. Pat. No. 3,792,286, where various interconnection arrangements are illustrated for two-phase displaced bridges driving two equally displaced load windings, to provide cancellation of the 5th and 7th harmonics of the output fundamental. In the Meier patent the corresponding phase outputs of the two bridges are relatively phase displaced by 30.degree. (360.degree./2PN, where P=3, N=2) and the load windings are similarly displaced to provide a unity power factor fundamental signal to the load. Cancellation of the 5th and 7th harmonics occurs, in a three-phase reactor connected to the phase outputs of each bridge, as a direct result of the summation of two (N=2) sets of three (P=3) phase corresponding fundamental signals separated relatively displaced in phase by the angle 360.degree./2PN=30.degree.. Cancellation for only two bridges is disclosed and any attempt to extend the teaching of Meier to inverters having more than two bridges results in the realization that the number of bridges must be limited to powers of two to allow for successive addition of pairs, then pairs of pairs and so on. Therefore, the next number of bridges required is four, with the signals of the second pair summed in a second reactor which cancels the 5th and 7th harmonics of the second pair, and with the outputs from each of the two reactors being summed in a third reactor which provides cancellation of the next two sets of higher order harmonics 11, 13, 17 and 19. Since it may be found practical to provide high power converters with as many as seven inverter bridges, Meier fails to teach a method or apparatus for cancelling the fundamental harmonics in a three, five, six or seven bridge converter system.
A second U.S. Pat. No. 3,876,923, to A. J. Humphrey et al provides an extension of the Meier teaching by disclosing the use of the Meier arrangement for a two inverter bridge system relatively displaced at the interpole angle of 30.degree. and providing three-phase power through a similar reactor into a phase displaced load winding, which in Humphrey et al is disclosed as a combination of phase and auxiliary windings in a common magnetic circuit as opposed to the electrically isolated loads of Meier. As in Meier, there is no suggestion by Humphrey et al of a method or arrangement for cancelling the fundamental harmonics in an inverter system having non-power of two numbers of bridges.
U.S. Pat. No. 4,204,264 assigned to the same assignee as the present invention provides harmonic signal cancellation in an inverter system having non-power of two numbers of bridges wherein the fundamental signals of each bridge are phase displaced by 60.degree./N degrees relative to the fundamental signals provided at another bridge. The bridges are grouped and fundamental signals are produced for each group which have a phase angle corresponding to one of the converter phases. The signal harmonics in one group of an order less than (6N-1) displaced 180 degrees from the equal order signal harmonics of another group. The fundamental signals having corresponding phase angles are added and provide the output fundamental signal in each of the converter phases with the signal having a first harmonic of order (6N-1).
Each of the foregoing patents disclose methods and apparatus which attempt to cancel harmonic signals in the output fundamental signals of multi-bridge, three-phase power converters. Typically, such approaches require balanced circuit arrangements and equal bridge current magnitudes which necessitate exact ratios which are a funciton of transcendental functions. Thus, large turns transformers with only restricted integer windings can be used. These transformers are generally very expensive and add greatly to the cost of power converters having such harmonic signal cancellation circuits. In addition, it is necessary to use transformers that have low flux levels in order to keep power losses down.
The transformers in high power inverters have only a few turns per winding despite having input voltages of several hundred volts, and windings having only a 1 or 2 per cent to exact ratios are feasible without auxiliary auto-transformers. In addition, winding unbalance may produce a 1 to 2 per cent difference in bridge loadings. Therefore, balanced impedances or exact sharing is not feasible or practical in practice to achieve exact harmonic cancellation and bridge loading.
It is therefore a general aim of the present invention to provide harmonic signal reduction for multi-bridge converters that produces an output voltage having an acceptable total harmonic distortion and single harmonic distortion for use in high power converters.
It is a further aim of the present invention to provide apparatus for harmonic reduction in output fundamental signals of a multi-bridge, three-phase converter for any combination of odd or even numbers of bridges.